The more conventional induction heating techniques use configurations which are satisfactory when the parts to be heated are always of the same type and of the same dimensions. However, industry increasingly requires flexibility and productivity. Production lines are required to adapt during continuous operation to the change in position or format of the parts to be heated, and to adapt the desired temperature profile according to this change.
Known technologies make it possible to have control of the heating per injected power zone, but the control of the temperature profile in the heated zones remains related to the geometric design of the coils and to their power supply method, principally by amplitude variation of the currents injected into them. The determination of these currents and the regulation resulting from this contributes greatly to the magnetic coupling existing between the coils because of mutual induction, each powered coil having an effect on all the others. Magnetic coupling makes the control of the temperature profile of the heated part extremely difficult, without considering that there can be harmful repercussions on the frequency generators, for example a breakdown of components.
Patent Application WO 00/28787 A1 describes a system for heating a tubular metal part by induction coils powered by the intermediary of a switching circuit of the dimmer type connected to a power supply source of the inverter type. A control circuit makes it possible to vary the duration of the power injected by the power supply source into each coil in order to heat different zones of the metal part differently in view of a desired temperature profile. The injection of power into a coil is therefore carried out in an “all or nothing” way, i.e. it can be prevented over a cycle corresponding to several periods of the inverter's signal. This system does however have drawbacks, and in particular it makes it possible to control only the average power produced by each coil without being able to control accurately the temperature profile generated by the coils in the heated part. Moreover, this document reveals that the connection of the coils and the inverters must be to a certain degree defined according to the load and to the temperature profile to be achieved. Furthermore, this document does not mention the magnetic couplings between the circuits or the way to be unaffected by them or to take them into account.